Field of the Invention
The invention lies in the field of semiconductor circuits. The invention relates to a circuit configuration for driving a semiconductor switching element, in particular, a semiconductor switching element that is connected up in a device for generating an ignition spark in an automobile.
Such a device for generating an ignition spark with a semiconductor switching element is illustrated in FIG. 1. The device has a transformer TR with a primary coil L1, a power transistor T, and a spark plug Z. The power transistor T is constructed, in particular, as an Insulated Gate Bipolar Transistor (IGBT) or as a so-called power Darlington bipolar stage and is connected in series with the primary coil L1 between two supply potentials V+, GND. The spark plug Z is connected in series with the secondary coil L2 of the transformer TR. Two diodes D1, D2 that are connected in series between the collector terminal and the gate terminal of the power transistor, configured as an IGBT in the example, serve for limiting the collector-emitter voltage of the transistor T by virtue of the transistor T being driven into the on state through these two diodes D1, D2 if the potential at the collector K rises above a predetermined amount.
If the power transistor T is driven into the on state by the application of a suitable drive potential at its gate electrode G, a current flows through the primary coil L1, as a result of which the primary coil L1 takes up energy. If the power transistor T is subsequently turned off, the primary coil induces a high voltage in the electric circuit of the primary coil, the high voltage, or the energy stored in the primary coil, being transmitted to the secondary side, where it leads to the generation of an ignition spark in the spark plug z.
In such devices, disturbance situations can occur in which the generation of an ignition spark is to be prevented under all circumstances, even when the power transistor T is already in the on state and the primary coil L1 has already taken up energy. In such a case, simply switching off the power transistor T would lead to the generation of an ignition spark.
To avoid an ignition spark in such disturbance situations, the prior art includes changing over the collector of the power transistor by suitable circuit measures and, thereby, put at a potential value at which no ignition spark is generated.
In the case of so-called intelligent power transistors fabricated using chip-on-chip technology, a problem arises that the collector of the power transistor is not accessible. In such technology, the power transistor is realized in one semiconductor body and a drive circuit, protective circuits of the transistor, and the like are realized in a second semiconductor body, which is fixed on the first semiconductor body.
Due to the diverse additional functions, in particular, due to integrated protective circuits that deploy in the event of a short circuit of the load, to protect the power transistor, endeavors are being made to be able to use intelligent power transistors, so-called smart FETS or smart IGBTs, also for devices for generating ignition sparks.
It is accordingly an object of the invention to provide a circuit configuration for driving a semiconductor switching element, preferably, a power transistor, in particular, a power transistor in a device for generating an ignition spark that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that ensures no ignition spark is generated when a disturbance situation occurs.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a circuit configuration for driving a semiconductor switching element, including an output terminal to be connected to the semiconductor switching element, a capacitive charge storage configuration coupled to the output terminal, a charging and discharging circuit having at least one input receiving at least one drive signal and an output connected to the capacitive charge storage configuration, the charging and discharging circuit providing, at the output, one of the group consisting of a charging current and a first discharging current for the capacitive charge storage configuration depending on the at least one drive signal, a discharging circuit having a connecting terminal connected to the capacitive charge storage configuration, and the connecting terminal providing a second discharging current for the charge storage configuration.
The capacitive charge storage configuration, which is connected to the output, serves for providing a drive potential for a semiconductor switching element, in particular, a power transistor, which can be connected to the output terminal. In such a case, the voltage across the charge storage configuration or the drive potential rises if the charge storage configuration takes up current from the charging and discharging circuit, and the voltage, or the drive potential, falls if the charge storage configuration is discharged.
In such a case, the charging and discharging circuit is configured such that it is able to charge the capacitive charge storage configuration comparatively quickly and also discharge it comparatively quickly. A power transistor connected to the output terminal is in the on state if the charge storage configuration is charged and it is in the off state if the charge storage configuration is discharged. In disturbance-free operation, the capacitive charge storage configuration is, therefore, charged rapidly through the charging and discharging circuit to drive the connected power transistor into the on state, and the capacitive charge storage configuration is discharged rapidly through the charging and discharging circuit to turn off the connected power transistor. In such a case, the time duration within which the charge storage configuration is discharged through the charging and discharging circuit, or within which the power transistor undergoes a transition from the on state to the off state, can be coordinated with the further connections of the power transistor and can be set to such a short time duration that when the power transistor is switched off, a sufficient voltage is induced in a connected primary coil of a transformer, so that an ignition spark is generated in an ignition coil connected to the secondary side of the transformer.
Preferably, in accordance with another feature of the invention, the discharging circuit provides a constant discharging current for the charge storage configuration, the discharging current being significantly lower than a discharging current of the charging and discharging circuit and also significantly lower than a charging current of the charging and discharging circuit. The discharging circuit, which draws current from the charge storage configuration, preferably, permanently, does not influence the functioning of the circuit configuration in disturbance-free operation, in which the charge storage configuration is alternately charged and discharged through the charging and discharging circuit.
When a disturbance situation occurs, the charging and discharging circuit can be driven by the drive signal such that it provides no current at its output terminal. In such a case, only the discharging circuit acts, which continuously draws current from the charge storage configuration until the latter is completely discharged. The discharging current is coordinated with the capacitance of the charge storage configuration such that the voltage change brought about by the discharging current across the charge storage configuration is so small that the power transistor connected to the output terminal undergoes a transition from an on state to an off state so slowly that an ignition spark is not generated in a spark plug connected on the secondary side.
By the circuit configuration according to the invention, the generation of an ignition spark can be prevented exclusively by driving the control terminal, that is to say, the gate terminal when using MOSFET or IGBT as semiconductor switching elements, by virtue of the power transistor being transferred very slowly from the on state to an off state when a disturbance situation occurs.
The capacitive charge storage configuration includes a capacitor in the simplest case. It is also possible to use any further charge storage configurations desired.
In accordance with a further feature of the invention, the discharging circuit has a bipolar transistor and a current source, the base of the bipolar transistor being connected to the capacitive charge storage configuration and the current source being connected to the emitter of the bipolar transistor. In the embodiment, the discharging current of the discharging circuit is the base-emitter current of the bipolar transistor. Such a discharging circuit can realize discharging currents in the nanoamperes range, thereby ensuring sufficiently slow discharging of the charge storage configuration and, hence, sufficiently slow turning-off of the semiconductor switch when a disturbance situation occurs.
In accordance with an added feature of the invention, the charging and discharging circuit has a first and a second controllable switch each having a control input, which are connected in series between a first and a second supply potential. In such a case, the first and second switches can be driven by a drive circuit to which the at least one drive signal is fed. The drive circuit is configured such that it drives the first and second switches complementarily in disturbance-free operation, that is to say, drives only in each case one of the two switches into the on state, in order thereby to connect the charge storage configuration either to the first supply potential or to the second supply potential, in order either to charge or to discharge the charge storage configuration. Furthermore, the drive circuit is configured to turn off both switches when a disturbance situation occurs to prevent current both from being output to the charge storage configuration and from being taken up by the charge storage configuration.
In accordance with an additional feature of the invention, the drive circuit has a first output terminal connected to the first control input and a second output terminal connected to the second control input.
In accordance with yet another feature of the invention, the drive circuit preferably has a first input terminal for feeding in a first drive signal and a second input terminal for feeding in a second drive signal, the first and second switches being driven complementarily according to the first drive signal during disturbance-free operation, and the second drive signal serving to indicate a disturbance situation and both switches being turned off if the second drive signal assumes a predetermined level that represents a disturbance situation.
In accordance with yet a further feature of the invention, the first controllable switch has an on state and an off state, the second controllable switch has an on state and an off state, and the drive circuit drives one switch of the group consisting of the first controllable switch and the second controllable switch into the on state and the other switch of the group consisting of the first controllable switch and the second controllable switch into the off state dependent upon the at least one drive signal.
In accordance with yet an added feature of the invention, the at least one drive signal includes a first drive signal and a second drive signal, the at least one input includes a first input and a second input, the drive circuit has a second input terminal connected to the second input for feeding in the second drive signal, and the drive circuit drives the first controllable switch and the second controllable switch jointly into the off state depending on the second drive signal.
In accordance with yet an additional feature of the invention, an operational amplifier is advantageously connected between the charge storage configuration and the output terminal of the circuit configuration. The operational amplifier preferably has a gain of 1 and ensures that the voltage across the charge storage configuration is also present at the control terminal of a connected power transistor. In such a case, the operational amplifier is necessary, in particular, to prevent the drive potential of the power transistor from rising above the potential at the charge storage configuration, for example, due to a Miller capacitance that may be present in the power transistor.
With the objects of the invention in view, there is also provided a method for driving a semiconductor switching element having a control input and a load path, including the steps of connecting the load path of the semiconductor switching element in series with a primary coil of a transformer, connecting an ignition spark generating configuration in series with a secondary coil of the transformer, applying a drive voltage to the control input of the semiconductor switching element to an extent sufficient to drive the semiconductor switching element into an on state, generating an ignition spark in the ignition configuration by reducing the drive voltage of the semiconductor switching element within a sufficient period of time to induce a voltage across the load path sufficient to generate an ignition spark in the ignition configuration, and preventing an ignition spark in the ignition configuration by reducing the drive voltage of the semiconductor switching element within a sufficient period of time to induce a voltage across the load path insufficient to generate an ignition spark in the ignition configuration.
To generate an ignition spark, the method provides for a drive voltage to be applied to the control input of the semiconductor switching element that is large enough to drive the semiconductor switch into the on state. To generate an ignition spark, the drive voltage of the semiconductor switching element is reduced, the drive voltage being changed so rapidly that the voltage induced across the load path suffices to generate an ignition spark in the ignition spark generating device on the secondary side of the transformer. If a disturbance situation occurs in which the generation of an ignition spark is to be prevented despite the semiconductor switching element already being in the on state, the method according to the invention provides for the drive voltage of the semiconductor switching element to be reduced so slowly that the voltage induced across the load path does not suffice to generate an ignition spark in the ignition spark generating device.
Such a method can be carried out by the circuit configuration according to the invention.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for driving a semiconductor switching element, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.